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Plastic Injection Molding

About 2 years ago I purchased a Tool set. It came with a wonderful set of tools, but a really low end, thin and hollow plastic injection molded case that belies the namesake and tradition. Needless to say, the case finally gave up the ghost this past week. I began shopping for a replacement tool box easy for the old set that has since grown to over 200 pieces. Wanting some level of organization that is needed on the trail while off roading, the plastic tool box intrigued me due to it’s thick plastic construction and it’s similarity to a larger tool chest with extendable drawers. Its size and depth allowed me to place it in the same spot as where I held the old plastic injection molded case in my truck.

Plastic Injection molding is preferred for producing complex shapes of various sizes having very fine details. The parts produced by using this method have greater dimensional control. Plastic Injection molding is used for producing parts in large quantities. Plastic molding is widely used for many applications. When you design parts for plastic injection molding, you can arrange multiple pieces in one mold by connecting them with small bars ~0.1″, however the sub-components must not have widely varying volumes. When the design does not allow for additional structures to improve strength, consider using a stronger material, such as glass fiber filled plastic. Consider specifying a fire retardant material when necessary.

Nowadays, liquid injection molding is becoming increasingly important. One reason for this is the increased performance requirements of the finished rubber parts. In addition, more and more producers of rubber parts are seeing the benefits in crown the high level of automation and productivity. Injection molding technology, using liquid rubber, combines the speed, cost efficiency, and versatility of plastic injection molding with the outstanding properties of silicone rubber. It is used in many applications where organic rubber cannot perform, such as military, aerospace, automotive, healthcare applications, etc. The use of silicone rubber combines the properties of resilience, high temperature stability and inertness as silicone is generally unaffected by temperatures from 100 degrees Fahrenheit to 450 degrees Fahrenheit. Progress in liquid injection molding technology has made faster cures possible, enabling unit cost reductions.

The Plastic injection molding machine has two parts: the injection unit which melts the plastic and injects the plastic molding into the mold, and the clamping unit easy which holds the mold. The injection molding unit clamps the mold in a closed position during injection, opens the mold after cooling, and ejects the finished part. Injection molding’s efficiency varies by the number of parts you plan to produce. For a small quantity, such as 10 parts, it may be less expensive to simply machine the desired part utilizing a milling machine. On the other hand, if you desire several thousand parts, injection molding is ideal. When you submit your order for an injection molding part, the websites will assign staff to help evaluate the most appropriate mold material which will vary by part tolerance and number of units anticipated.

Electrical Discharge Machining And Mold Making

   The use of electrical discharge machining is so essential that it is almost impossible to imagine a modern shop without an EDM machine. Many of today’s products simply could not be produced without it. Think of all the cell phones, calculators, IPods, cameras, medical devices and the endless amounts of high tech equipment that are made out of plastic!

Besides the essential role it plays in plastic injection mold making, EDM is used for the precision machining of medical parts, aerospace parts, and other highly specialized products. Electrical discharge machining, including Wire EDM, has also completely changed the way molds are made, as well as other specialized parts.

This is your information source for the EDM die sinker

Graphite  Copper EDM Tooling EDM Oil
EDM Filters Specialty Electrodes Carbide EDMing Surface Finishes

Check out the pages dealing with graphite, graphite dust, copper, dielectric fluid, electrode holders, and machinery.

What about jobs in EDM?

Find out what kind of personality it takes to have electrical discharge machining as a career job. See what the future holds in this high-tech profession.

You can also find educational material that attempts to explain this rather mysterious process. Not so long ago, it was as much black art as science. Today, however, much of the mystery is removed due to CNC controls.

Most shops send their operators to classes put on by the manufacturer of their EDM machine. It is foolish to invest so much money in a machine and skimp on the training! But it happens all the time.

What about EDM and stress?

EDM is quite a stressful occupation, to put it mildly. It may not appear difficult, but the stress levels can be exhausting. Nearly every mold component that is EDM’d is in the final stages of manufacturing. The slightest mistake can become very costly, very quickly.

There is nothing quite like the feeling of coming to work in the morning and seeing your supervisor leaning over the EDM machine staring at your workpiece. Then you find out that you had mixed up two locations and the completely finished cavity block is now completely useless!

Injection Mold Design

Plastic injection molding for producing plastic products

Plastic injection molding is the principal process for producing plastic products or parts of products. Plastic is acknowledged to be a very flexible and cost-effective material that is used in many applications. Although the tooling can be expensive, the cost, per part is very low. Intricate geometries are limited only to the abiltiy to create the injection mold. Things you use everyday, such as the case that houses your monitor, the keyboard on which you type or the mouse on your desk were produced with plastic injection molding.

Plastic injection molding involves the transformation of a plastic solid, usually in the form of granules or pellets, and heating the plastic resin to a specific temperature until it melts. The melt is then forced into a mold made up of two or more dies, where it is forced to “cool”, resulting in producing the desired shape. A specific amount of time passes, usually a few seconds, and the mold is then opened and the part is released. This cycle then repeats continuously until the desired quantity is reached.

Considerations of  Injection Mold Design

The design of the part, and therefore the mold, needs to include draft features (angled surfaces) to make possible the removal of the part from the mold. Typical draft angles are about 1 to 2 degrees for part surfaces which do not exceed five inches. Dimensional tolerance specification will dictate the final cost of the part as well as its ability to be manufactured. If there is a small section of the part which needs higher tolerances, such as the location of a critical feature used for alignment,do not specify a tight tolerance, as an alternative, plan and design for post molding processes such as machining to achieve the desired results.

Radii and Corners

It is very important that uniform wall thickness be maintained at the corners. The internal and external radius need to share the same center point. External radii = internal radii + wall thickness. The minimum radii should not be less than ¼ of the minimum wall thickness. Design for radii to be ½ to ¾ of the nominal wall thickness. When a large amount of stress is going to be present, it is very important to design in larger radius as this will distribute the stress much more evenly.

Wall Thickness

The production of thin wall items such as a clamshell for retail packaging are possible with today’s technology. Products with thick walls are also easily produced. However, parts which require uneven wall thickness present a challenge to the plastic molder manufacturer. Creating a part with a uniform wall thickness and cross section will abridge manufacturing and reduce costs. One issue to be aware of is sinking. Wherever an intersection or “tee” occurs, there will be some degree of sinking. This occurs because thicker walls cool at a slower rate and therefore create this problem.

Ribs

Ribbing should be ½ to two thirds of the nominal wall thickness and less than 3 times the thickness in height.A taper of 1° is usual. Note: as mentioned above, excess thickness can result in shrinkage.An excess in rib height combined with a taper will produce thin areas requiring extra fill time at the mold.

Weld (Part) lines

The location of weld lines needs to be considered by designer before a injection mold is created. Weld lines are formed by the joining of the flow fronts of the plastic during molding. One issue of concern is the that the weld line area is more susceptible to cracks and stress failure.

Bosses

  • Diameter = (Outside Diameter) \ (Inside Diameter) = 2 to 3
  • Thickness = 1/2 to 2/3 nominal wall thickness
  • Gusset Height = 2/3 Height
  • Height = Fastener minimum requirements
  • Taper = 1 deg. all around
  • Diameter Ratio should be minimum ratio of 2., this will reduce risk of failure.

Pressure:

Another factor in the design will be the clamping pressure required to produce the part while the plastic is being injected. Smaller cavities can result in high pressures being required to force the plastic or rubber material to fully fill the mold cavity. This will, in turn, determine the thickness of the mold material, usually steel) as well as the type of machine in which can be used.

Summary:

Many factors must be taken into account when designing a mold for the creation of plastic injection molded parts. Factors such as draft angles, wall thickness, ribbing (not the kidding kind), bosses and weld lines and clamping pressure all come into play when designing a mold that will be used in a plastic injection mold machine. Each facet is important in and of itself, but as a whole, each one affects the others. Therefore the design of a mold for plastic molding can be quite involved. When done correctly, the result will be a mold which will yield thousands, hundreds of thousands, or even millions of parts over it’s lifetime.

Injection Molding Companies

Injection Molding Companies

These are some of the top companies in the United States in the injection molding field. If you are looking for a great company with great reputations, check out these companies.

MOCAP, Inc, located in St. Louis, MO. They are a full service company; they are a manufacturer of standard and custom injection molding designs. They use all grades of plastic and they do a lot of volume and can handle as little or as much volume as needed.

Precision Tool & Die located in Derry, New Hampshire. They are an all purpose company, they provide custom manufacturing, service and a general manufacturer. They provide on site molding with perfect injection molds, they also do insert molds, transfer molds and they also do complete designs.

World Class Plastics located in Russell Point, Ohio, this company is also a custom manufacturer, a service and a manufacture company. Their company profile says they are QS9000 registered, they do custom thermoplastic injections, and they fabricate molds and tool design.

The G&F Industries located in Sturbridge, MA, they are a service and manufacture company. They are listed as an ISO 9000 certified company; they have a great reputation in the fabrication of injection Plastic Molding and plastic fabrications that use the injection molding process. They also use a robotic system and a real time production team.

Do you know any other injection molding companies that deserve to be mentioned here?

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